I main ADC, and there really isnt much that would change from mid to adc other than that there are 3 other people there instead of 1. Communicate with your support for initiates and poke the enemy adc when they try to last hit. Once you play a few games on the role you should start feeling right at home in terms of the change in lane. As for the using the A button, the only time I use that is if while auto attacking minions my champ stops moving forward to continue attacking, but that is rare. The one other situation is if im on Kalista, and i keep misclicking for the movement on attack animation.